The present invention relates to a logic system based controller and more specifically a controller that utilizes state machines, logic and/or a microprocessor for an electro-optic system.
Electro-optic systems are known in the art for providing an interface between electronic and optically-based systems. Such electro-optic systems are used in a variety of applications including telecommunications, remote sensing, medical devices, and in other fields as well.
FIG. 1a illustrates a conventional electro-optic system 100 for use with one of the aforementioned systems. The electro-optic system 100 includes a laser 110 and an analog bias controller 120. Typically, an electronic system produces a modulating signal which is combined 112 with a laser bias current IDC. The combined signal is injected into the laser 110 which produces a modulating optical output signal, PO, that typically is an intensity modulated signal in which the light intensity varies as a function of the amplitude of the modulating signal. In analog systems, the bias current IDC is selected such that it biases the laser 110 at an operating point where the laser output PO exhibits a linear relationship to the input bias IDC (herein referred to as the laser""s transfer function). The modulating signal varies the bias point above and below this operating point thereby producing a corresponding change in the intensity of the output power, PO, of the laser 110.
Continuing with FIG. 1, the laser 110 may include a monitoring photodiode (not shown) either separate or integrated into the same package with the laser 110. The monitoring photodiode produces a photodiode current IPD indicative of the laser output power Po. The photodiode current IPD is supplied to an analog bias controller 120 which includes circuitry designed to measure the photodiode current IPD and to return the laser bias IDC to a predefined current level.
FIG. 1b illustrates a graph showing three transfer functions of a laser at three operating temperatures T1, T2, T3. The graph illustrates laser output power PO along the y-axis, and laser bias current IDC along the x-axis. The first trace 152 illustrates the laser""s transfer function at a first operating temperature T1. The second trace 154 and the third trace 156 similarly illustrate the laser""s transfer function at a second and a third operating temperature, T2 and T3, respectively. Each of the transfer functions 152, 154, and 156 also include a corresponding threshold operating point Ith1, Ith2, and Ith3, respectively. The threshold operating points Ith1, Ith2, and Ith3, indicate the laser bias current level IDC at which the laser produces appreciable output power. As can be seen, a laser""s threshold operating point varies greatly with operating temperature. A change in the operating temperature of the laser shifts the transfer function laterally along the x-axis.
A shift in the operating temperature can produce a significant change in the laser output power. For example, a laser bias current IDC selected to bias the laser at operating point IQ1 produces a laser output power PT1 in the center 162 of the linear region of the transfer function 152. At operating point IQ1, The laser will operate as intended to produce a substantially linearly varying output power PO when excited by a modulating signal. If the operating temperature of the laser operating temperature changes to T2, the laser output power PT2 for the same bias current IQ1 drops significantly as shown by operating point 163. In this case, the linear operating region of the laser and electro-optic system 100 is limited to the linear region below operating point 163, in the case of negative current swings below IQ1. The analog bias controller 120 contains circuits to compensate for change in temperature, but with a limited degree of accuracy. The analog bias controller 120 also requires manual adjustments, has limited control capabilities, and is subject to significant temperature drift.
In view of the aforementioned inadequacies of the prior art, the need exists for a new system and method for controlling an electro-optic system over varying temperatures of operation.
It is an advantage of the present invention to provide a system and method for controlling an electro-optic system by which the threshold operating point of the system gains and other parameters can be accurately established and the laser transfer function controlled over a variety of different operating temperatures of a laser module.
It is another advantage to provide a system for subtracting errors due to dark current and aging of a laser-monitoring photodiode.
It is yet another advantage to provide a system for driving a laser utilizing both a low frequency circuit to drive DC current in addition to a high frequency circuit for driving a high frequency AC/DC signal to set the operating point of a laser.
A further advantage is to provide a system and method for calibrating and initializing the electro-optic system upon each power up of the system.
It is another advantage to provide a system for continuously monitoring and adjusting parameters of an electro-optic system while the system is fully operational.
Still another advantage of the present invention further is to provide a system and method for enabling a link characterization process to be performed between two electro-optic systems.
Electro-Optic System Controller
In an exemplary embodiment, the present invention is an electro-optic system for driving a laser, monitoring the laser operation, and maintaining the laser parameters within acceptable limits over temperature and device variations. The electro-optic system of an exemplary embodiment includes a signal input coupled to a pre-amplifier via a test system switch (TSS). The output of the preamplifier is fed into a combiner and coupled to a laser module input. A laser module of an exemplary embodiment includes a laser bias input, a laser output, a temperature sensor output, and a monitoring photodiode sensor output. In other embodiments, the laser module may further include a power sensor and a cooler/heater component. The test system switch, the preamplifier and the laser module are controlled by a controller circuit.
The controller circuit of an exemplary embodiment may be a digital processor, a microprocessor, or an ASIC device that is capable of generating control data. The processor of the exemplary embodiment includes a communication port that may be connected to an external host computer. The processor of the exemplary embodiment further includes memory or an input for connection to a memory component. In other embodiments of the present invention the processor may include a port for storing/receiving digital data to/from an external source.
The processor controls the test system switch utilizing switch control lines. In an exemplary embodiment, the test system switch may connect one of a data signal, a signal generated from a signal generator, or a reference ground to the input of the preamplifier. The signal generator is controlled by the processor. The test system switch of an exemplary embodiment also includes a switched or dedicated line for connecting a coded input signal to a code detector of the controller circuit. The code detector identifies the reception of an encoded signal received by the electro-optic system and outputs coded data to the processor.
The preamplifier of an exemplary embodiment includes an amplifier connected to the switched input from the test system switch, a high frequency voltage variable resistor, and a high frequency voltage controlled current source (HF-VCCS) that is connected to the signal input of the laser. The preamplifier is coupled to the processor and may have its gain and offset characteristics changed by means of a dual digital to analog converter (DAC) that accepts digital control signals from the processor. The dual DAC of an exemplary embodiment dedicates one DAC for adjusting the gain of the preamplifier and a second DAC for adjusting the offset of the preamplifier.
The driving signal to the laser input of an exemplary embodiment is the combination of a high frequency DC/AC signal current IS and a low frequency laser bias current IL. The biasing current IL is generated by the controller of the electro-optic system utilizing a low frequency voltage controlled current source controller by the processor by means of a bias adjust DAC. The controller of an exemplary embodiment may further include digital to analog converters connected to the processor for controlling the wavelength of the laser, and for controlling other laser components such as a cooler/heater.
The controller of an exemplary embodiment of the present invention further provides circuitry for monitoring the laser module components including the photodiode and the temperature sensor, the preamplifier, and also includes other signals generated by the controller such as the low frequency bias current IL. In an exemplary embodiment, a voltage signal is provided by the temperature sensor that is indicative of the real-time temperature of the laser module. This temperature voltage signal is fed into a sensing amplifier of a package of sensing amplifiers. The outputs of the sensing amplifiers are connected to a multiplexer that is coupled to the processor via an analog to digital converter. Other signals are also supplied to the processor by means of the sensing amplifiers including a test signal from the high frequency VCCS of the preamplifier and a test signal from the low frequency VCCS. The photodiode of the laser module of the exemplary embodiment includes a photodiode current output signal that is connected to a photodiode amplifier in the controller circuit. The amplifier is controller by the processor via a dual digital to analog converter that includes an offset adjust DAC and a gain adjust DAC. The output of the photodiode amplifier is connected to an input of the multiplexer. The multiplexer of the exemplary embodiment also includes additional inputs for monitoring the various voltages utilized by the electro-optic system.
Factory Calibration
An exemplary embodiment of the present invention provides a factory calibration process for calibrating the laser module system. Test equipment utilized for the laser calibration is connected from the laser output to the laser drive control system. In one embodiment, the testing equipment may consist of an external optical power meter instrument having an I/O interface to a host computer. Once the testing equipment is connected to the laser module system, the dark current of the laser module photodiode is determined by first setting the gain of the laser preamplifier to nominal. The Test System Switch (TSS), which is connected to the input of the laser preamplifier, is connected to a circuit ground utilizing a control signal from the microprocessor of the electro-optic system.
The calibration process of the exemplary embodiment continues by turning off the DC bias for the laser by supplying an appropriate digital word to a bias adjust digital to analog converter (DAC). The analog signal produced by the bias adjust DAC turns off the current normally generated by the Low Frequency Voltage Controlled Current Source (VCCS). The offset voltage of the preamplifier is canceled by adjusting an offset adjust DAC while monitoring the offset utilizing a measurement of the voltage across a resistor of the high frequency VCCS. The dark current of the laser photodiode is calibrated to avoid errors as the laser photodiode changes with temperature and aging. If the laser has no current drive, then the output light power of the laser is zero, that is, no light is coupled into the monitoring laser photodiode. The parasitic dark current ID is defined as the current that flows through the laser photodiode for a no-light output condition. The measured dark current is recorded and utilized to compensate the system measurements.
The threshold currents of the laser, and corresponding laser photodiode currents are determined in the next steps of the exemplary calibration method. The DC bias current to the laser IL is incremented gradually by changing the input of the bias adjust DAC utilizing the microprocessor control lines. The change to the input of the bias adjust DAC changes the current output of the low frequency VCCS on the DC bias path. At the same time, laser power PL is read with the external optical power meter, and the microprocessor computes the slope of the transfer characteristic dPL/dIL. In the next step of the exemplary embodiment, the laser bias current IL is set to a threshold ILTH by setting the bias current IL to a value at which the derivative dPL/dIL stops changing. This step sets the threshold ILTH in the linear region of the laser characteristic. Once the threshold ILTH is determined, the laser power at threshold PLTH is read utilizing the optical power meter and recorded by the microprocessor.
The calibration method of the exemplary embodiment continues by measuring the laser photodiode current IP1 under the above threshold conditions. Specifically, the threshold values of the laser photodiode current IP1T are measured and recorded via a path through a photodiode amplifier to a MUX and analog to digital (A/D) converter that is connected to the microprocessor. The laser threshold current ILTH is measured and recorded via a path through a measurement resistor to the sensing amplifiers, through the MUX and A/D converter, and to the microprocessor. The recorded value of the laser photodiode threshold current IP1T is adjusted utilizing the value of the dark current ID. A precision method for measuring the laser bias current IL starts with the sensing of a voltage across a resistor of the low frequency VCCS utilizing a sensing resistor. The sensed voltage is routed through an amplifier of the sensing amplifiers to the multiplexer, and is converted to a digital word and routed to the microprocessor by the A/D converter. The temperature of laser module is read and recorded at the threshold values so that the microprocessor may associate a given temperature to the laser threshold current ILTH, to a photodiode threshold current IP1T, and to photodiode dark current ID.
Continuing with the calibration process of an exemplary embodiment, the input to the laser preamplifier is disconnected from circuit ground and connected to a voltage reference of a signal generator utilizing the test system switch. The voltage reference utilized in the calibration process is a precision full-scale reference voltage VREF that is applied to the input of the laser preamplifier. The gain G1 of the laser preamplifier is incremented while reading the power output PL of the laser utilizing the optical power meter. The gain G1 of the laser preamplifier is incremented by the microprocessor by applying an appropriate digital word to a gain adjust DAC. The gain adjust DAC, in turn, applies a voltage to the high frequency voltage variable resistor, which then changes the gain G1 of the laser preamplifier. The gain adjust DAC is incremented to determine a full power gain GF needed to obtain full-scale laser power PLMI. The maximum current IPM through the laser photodiode is determined for the full-scale laser power PLMI condition.
The effective photodiode responsivity Reff of the combination of the laser module and the laser photodiode is calculated and recorded. The factory calibration process of the exemplary embodiment is completed by preparing drift compensation models, storing the models in a processor memory, and disconnecting the test equipment utilized for the laser calibration.
Power Up Calibration and Initialization
An exemplary embodiment of the present invention provides a power up calibration process for calibrating the laser module system. Upon power up of the laser module system, the preamplifier gain is set to the to the full power gain GF1 obtained during factory calibration, needed to obtain full-scale laser power PLMI gain. The input of the laser preamplifier is connected to circuit ground using the test system switch. In a next step, the DC bias for laser is turned off by supplying an appropriate digital word to a bias adjust digital to analog converter (DAC). The offset voltage of the preamplifier is canceled by adjusting an offset adjust DAC while monitoring the offset utilizing a measurement of the voltage across a resistor of the high frequency VCCS. The dark current ID from the laser photodiode is measured and recorded. The temperature T of laser module is then measured and recorded.
The power up calibration and initialization process of an exemplary embodiment continues by determining the laser power threshold. The DC bias current IL to the laser is measured concurrently with the laser photodiode current IP1. The microprocessor then utilizes the recorded values from the above measurements to compute the laser power PL. The DC bias current IL is set to threshold by selecting a point at which dPL/dIL stops changing. In a next step of the exemplary embodiment, the laser threshold power PLTH is determined by reading the laser photodiode threshold current IPTH and computing and recording the corresponding laser threshold power PLTH. The laser threshold current ILTH and the photodiode threshold currents IP1T are also recorded. The process of determining the laser power threshold is completed by determining and recording the threshold temperature of laser module TTH.
The exemplary power up calibration process continues by calibrating the system gain. Utilizing the test system switch (TSS), the laser preamplifier input is disconnected from the circuit ground, and is connected to a precision full-scale reference voltage VREF of the signal generator. A photodiode current IPM resulting from the application of the reference voltage VREF is measured, and the corresponding laser power output PL is determined. In a next step, the microprocessor compensates for gain errors of the laser module. The gain G1 of the laser preamplifier is incremented while determining the laser power output PL with laser photodiode measurements. The gain GF needed to obtain full scale output laser power PLM is determined. The TSS path to the laser preamplifier is switched from the reference voltage VREF to circuit ground. The DC bias current is set to an a xe2x80x9cQ pointxe2x80x9d current ILQ, defined as an operating point above the threshold current, using the DC bias path measurement circuit. The microprocessor of the Electro-Optic System computes drift compensation for the above measured parameter. The power up calibration and initialization is completed by connecting the input of the preamplifier to the signal driving the laser.
Electro-Optic System Monitoring
In an exemplary embodiment, the Electro-Optic System enters a monitor phase where drift compensation is implemented on a continuous basis without having to disconnect the Electro-Optic System from the input signal. Adjustments to the system parameters such as laser DC bias or system gain are made as needed to compensate for temperature changes. The drift compensation process determines the adjustments with the use of drift models and then changes the system parameters, as needed, utilizing very gradual steps that emulate analog drift.